<!DOCTYPE html>
<html lang="en">
<head>
    <style>
	.container {
        display: flex;
	}
	.main {
        flex-grow: 1;
	    height: 300px;
	    background-color: red;
	}
	.left {
	    order: -1;
	    flex: 0 1 200px;
	    margin-right: 20px;
	    height: 300px;
	    background-color: blue;
	}
	.right {
	    flex: 0 1 100px;
        margin-left: 20px;
	    height: 300px;
	    background-color: green;
	}
    </style>
</head>
<body>
    <div class="container">
	<div class="main"></div>
	<div class="left"></div>
	<div class="right"></div>
    </div>
</body>
</html>